Conventionally used as a high-speed motor is, for example, an inverter-controlled inductance motor or a semiconductor motor (a brushless motor) including a small-inductance coreless motor. However, this kind of motor is generally equipped with a DC electric power source device for rectifying an output from an AC electric power source, so that the motor tends to become bulky and costly. Especially, where an inverter is built in a motor, such a disadvantage becomes more conspicuous.
Furthermore, the above-described conventional motor normally has a maximum speed of 6,000.about.10,000 rpm, but this maximum speed has to be decreased in order to obtain a large output. However, if a voltage applied to the motor is increased in order to increase its rotational speed, the efficiency of the motor will fall, and thus its practical merit will be lost.
Besides, at present, an electric circuit for constant-torque drive or constant-speed drive or servo control of the motor, which is capable of operating stably at a high-speed driving condition, is not available. Especially, one capable of operating with a high efficiency is not available.